Many vehicles are used over a wide range of vehicle speeds, including both forward and reverse movement. Some types of engines, however, are capable of operating efficiently only within a narrow range of speeds. Consequently, transmissions capable of efficiently transmitting power at a variety of speed ratios are frequently employed. When the vehicle is at low speed, the transmission is usually operated at a high speed ratio such that it multiplies the engine torque for improved acceleration. At high vehicle speed, operating the transmission at a low speed ratio permits an engine speed associated with quiet, fuel efficient cruising.
Typically, a transmission has a housing mounted to the vehicle structure, an input shaft driven by an engine crankshaft, and an output shaft driving the vehicle wheels, often via a differential assembly which permits the left and right wheel to rotate at slightly different speeds as the vehicle turns. A number of gearing elements, such as planetary gear sets, are interconnected with the input shaft, output shaft, and housing by a number of shift elements such as brakes and clutches. Various transmission speed ratios are established by selectively engaging various subsets of the shift elements.
One common type of shift element is a hydraulically actuated multi-plate wet friction clutch. A clutch pack includes a number of friction plates having friction material interleaved with a number of separator plates. The friction plates are splined to one of the elements to be selectively coupled by the shift element and the separator plates are splined to the other element. To engage the clutch, hydraulic pressure is applied to a piston which, in turn, applies axial force to a separator plate on one end of the clutch pack. The separator plate on the opposite end of the clutch pack is called the reaction plate. The reaction plate is held axially to provide a reaction force such that the force from the piston squeezes the friction plates between the separator plates. When the hydraulic pressure is released, a return spring pushes the piston away from the clutch pack to disengage the shift element.